Corrosion and impact resistant article and method of making same



nited States atent. O

CORROSION AND IMPACT RESISTANT ARTICLE AND METHOD OF MAKING SAlVIE PercyP. Turner, Jr., and Robert R. Ruppender, Euclid, Ohio, assignors toThompson Products, Inc., Cleveland, Ohio, a corporation of Ohio NoDrawing. Application January 24, 1952, Serial No. 268,146

5 Claims. (Cl. 29-198) The present invention relates to an impactresistant articlejconsisting of a refractory metal base and an impactresistant coating.

The instant application is a continuation-in-part of our application,Serial No. 214,116, filed March 6, 1951, entitled Corrosion and ImpactResistant Article and Method of Making Same.

As explained in the previously identified application, the extensivedevelopment in the field of jet engines has necessitated the developmentof alloys for use in the manufacture of parts for such jet engines whichcan withstand the extremely high temperatures and oxidizing atmospheresnormally present in the operation of turbojet engines. Such alloys, tofunction properly, must have a high strength, toughness,creep-resistance and resistance to the oxidizing gases present in theturbine engine.

One relatively plentiful metal which exhibits good strength, toughnessand creep-resistance characteristics is molybdenum. However, theoxidation resistance of molybdenum and alloys containing major amountsof molybdenum is quite poor. Although molybdenum has a melting point inexcess of 4500 F., it begins oxidation at temperatures as low as 900 F.The rate of formation of the molybdenum oxide increases with an increasein temperature. Since the molybdenum oxide sublimes, completedisintegration of the molybdenum body will occur in a relatively shorttime under conditions of high temperature oxidation.

To protect molybdenum bodies against the eflects of oxidation, it hasbeen suggested that certain metals or metalloids be coated onto thesurface of the molybdenum. Certain coating materials have the ability offorming intermetallic corrosion resistant compounds with the molybdenumsurface. In particular, coating materials such as silicon, aluminum, andzirconium form intermetallic compounds with molybdenum. Molybdenumbodies coated with silicon, for example, can withstand thousands ofhours of operation above temperatures of red heat without showing anyevidence of oxidation of the molybdenum base.

While these intermetallic compound coatings very effectively protect thesurface of the molybdenum from oxidation, the intermetallic compoundsformed at the surface of the molybdenum are usually quite brittle andliable to failure due to impact. Such impact could result during theoperation of gas turbine engines in aircraft due to the presence offoreign objects, even small particles, in the gas streams of the engine.These foreign objects may be received from the outside air, or mayresult from chipping of the combustion tubes, or from a number of othersources. These particles pass through the turbine at extremely highvelocities, on the order of several hundred feet a second. When theforeign objects strike the coated molybdenum part, fracture of thecoated surface often results, thereby exposing the refractory metal bodyto the corrosive atmosphere of the gas stream.

In our aforementioned application, we disclosed that brittle, corrosionresistant surfaces of the type described could be protected againstimpact by coating the surface with certain nickel-chromium alloys.

We have now found that substantially improved results can be obtained inpracticing the invention described in our aforementioned application byusing a plurality of nickel-chromium alloys of differing melting points,but having compositions within the ranges previously described. :Moreparticularly, we have found that if the nickel-chromium alloy coating isapplied to the surface of the refractory metal article in the form of amatrix of a liquefied nickel-chromium alloy containing dispersedparticles of another nickel-chromium alloy having a higher melting pointthan the matrix, that a superior coating is obtained as compared withthe results when a single alloy powder is used. The advantages areparticularly marked with respect to the maintenance of a uniform flow ofthe coating materials at the brazing temperature employed to apply thenickel-chromium alloy to the surface of the refractory body, and theability to apply thin coatings without the danger of discontinuities. 1

An object of the present invention is to provide an improved impactresistant coating to a refractory metal body.

Another object of the present invention is to provide a brazed coatingof avnickel-chromium alloy composition in the form of a relatively thinfilm about a corrosion resistant metal body.

Still another object of the present invention is to provide an improvednickel-chromium alloy composition capable of being brazed onto thesurface of a refractory metal body in the form of a uniform coating.

Still another object of the present invention is to provide an improvedmethod for applying impact resistant coatings to refractory metalbodies.

Another object of the present invention, is to provide a method forbrazing nickel-chromium alloy compositions onto refractory metalarticles.

In the method of the present invention, a refractory metal body isprovided with an impact resistant coating by applying a coatingcomposition consisting essentially of a liquefied matrix of a nickelchromium alloy, the matrix containing dispersed particles of anickel-chromium alloy of higher melting point than the melting point ofthe matrix.

In a preferred embodiment of the present invention, a mechanical mixtureof two nickel-chromium alloys having differing melting points, bothpowders having a particle size on the order of minus 325 mesh, issprayed onto the surface of the refractory article to be coated. Theresulting coated article is then heated to a firing temperaturesufficient to just melt the lower melting a1- loy but insuflicient tomelt the higher melting alloy. Under these conditions, the highermelting particles then begin to dissolve in the liquid phase, the extentof solution being dependent upon the time of treatment. We have foundthat the best coatings under these conditions are obtained byinterrupting the firing cycle before the point of complete solution ofthe higher melting alloy in the lower melting alloy matrix is reached.While this dissolution is taking place, the semi-fluid coating reactswith the base metal or object being coated, resulting in the formationof a small amount of higher melting alloy at theinterface which preventsor hinders the flow of the alloy coating at the given temperature. Theresulting coating is then uniform, tight, and integrally bonded to thebase metal.

The coatings of the present invention can be applied directly to arefractory metal body such as molybdenum, or alloys of molybdenumcontaining at least molybdenum. In addition, the alloy coatings of thepresent invention can also be applied to refractory metal bodies whichhave been treated, as by reaction with silicon, aluminum, or zirconium,to produce an outer corrosion resistant layer containing intermetalliccompounds of molybdenum and the coating metal.

As another alternative, the molybdenum base may be chromized for thedual purpose of rendering the base metal more oxidation resistant, andto use the chromium as a cushion or buffer layer between the base metaland the brazed coating. Chromium forms a series of solid solutions withmolybdenum, but not intermet'allic compounds. Furthermore, the brazedcoating, when applied to a chromized surface, is more uniform than it iswhen applied to bare molybdenum since the brazed layer reacts With thechromized layer and tends to eliminate uneven flow during brazing. Thechromizing step may be carried out by packing the article to be coatedin a layer of granular aluminum oxide which is subsequently surroundedby a pack containing chromium-bearing material such as chromium orferrochrome. Hydrogen and hydrogen-chloride gas are passed through thepack while maintaining the article at a temperature from 1800 to 2400"F. For the purposes of the present invention the chromizing is continuedfor a time, usually on the order of two hours, until a chromized coatingof from 0.0002 inch to 0.0004 inch is achieved. For the purposes of thisinvention, the application of chromium =by chromizing is definitelysuperior to the application of chromium by electrodeposition. Inchromizing, a substantial thickness of chromium can be built up, andbeing deposited from a vapor phase reaction, the chromium diffuses intoand reacts with the molybdenum base to form a definite bond with thebase metal. In addition, chromium deposited from a chromizing process ismore reactive than chromium deposited by other processes, and reactswith the subsequently applied brazed coatings.

The nickel-chromium alloys employed in the present invention preferablyhave analysis within the following ranges:

Mn 1.0 max. Si 0.5 to 5.0 Fe "a 1.0to 5.5 CI 8.0 to 20.0 B 1.0 to 8.0 Ni65 to 90 Zr 0.05 max. Ca 0.20 max. C 0.70 max.

The amount of carbon in the alloy is an important consideration, ascarbon tends to combine with chromium to form chromium carbide, therebyreducing the effective chromium content. The reduction in effectivechromium content reduces the oxidation resistance of the alloy at hightemperatures.

The silicon in the above composition may be added as ferro-silicon, andserves to increase the high temperature resistance to oxidation.

Iron imparts fluidity to the alloy and renders it more workable. Ironmay be added in the form of ferrosilicon, and this metal is also presentin some of the other ingredients as an impurity.

The boron content of the alloy not only lowers the melting point of thealloy but also increases the fluidity of the molten alloy by decreasingits surface tension.

Manganese occurs as an unavoidable impurity and lowers the hightemperature resistance of the alloy. The first tenths of one percent arethe most' detrimental in this respect.

Calcium and zirconium are added to the alloy to act as deoxidizingagents. The more completely deoxided the alloy, the less susceptible itis to precipitation of oxides at the grain boundaries of the alloy.These two elements serve to increase the high temperature resistance ofthe alloy.

A preferred coating process consists in mixing two nickel-chromium alloypowders, both having a particle size of minus 325 mesh, and spraying thepowdered mix- Alloy N0. 1

Alloy No. 2

The remainder of the analyses consist of unavoidable impurities.

We prefer to use powdered mixtures containing 40 to of alloy No. 1 with60 to 40% of alloy No. 2, and specificaly about 60% alloy No. 1 and 40%alloy No. 2

After application of the powdered metal to the surface of the refractoryarticle, the coated article is dried to eliminate free moisture and thenheated in a nonoxidizing atmosphere to braze the coating onto thecorrosion resistant surface. This brazing operation may be carried outin a dry hydrogen atmosphere, or under vacuum conditions, for a periodup to minutes at temperatures from about 1800 to 2300 F. For the bestresults, we have found that the brazing temperature should be chosen sothat it slightly exceeds the melting point of the lower melting alloy.Some of the discrete particles of the higher melting alloy are dissolvedin the liquefied matrix, while others float in the liquid matrix. Theliquid matrix reacts with the refractory metal body to form a highmelting alloy at the interface between the refractory metal surface andthe coating, thereby preventing or hindering the flow of the alloycoating. The tenacious bond between the brazed coating and theunderlying bare molybdenum, chromized molybdenum, or siliconizedmolybdenum, is due to this reaction between the coating and the base.

In applying these coatings to a molybdenum surface containing an inertoxidation resistant layer, such as a silicide of molybdenum, it isadvisable to etch the surface of the article with a reagent such ashydrofluoric acid to make the surface more receptive to the appliedcoating. A suitable etchant consists of one part hydrofluoric acid(4555%) with two parts water, the etchant being applied for a period offrom 10 to 120 seconds. In the case of silicide coatings, an etchingtime of about 40 seconds is used while for a chromized coating, anetching time of about 15 seconds is employed.

For best results, the coating should be applied so that the finalthickness of the coating is within the range from 0.002 inch to 0.010inch in thickness, with 0.0030 to 0.0040 inch thickness being an optimumrange.

The coatings produced according to the present invention are extremelyuniform, smooth and ductile. Surprisingly, the same results are notachieved when using a single alloy having a composition corresponding tothe average composition of the mixture of the two alloys.

From the foregoing, it will be appreciated that the novel coatings ofthe present invention provide increased corrosion and impact resistanceto refractory metal articles, making them particularly adaptable to usein high temperature environments and oxidizing conditions, such as occurin turbo-jet engines. The coatings are securely bonded to the underlyingrefractory metal base, are extremely uniform, smooth and ductile.

Mn 1.0 max.

Si 0.5 to 5.0 Fe 1.0 to 5.5 Cr 8.0 to 20.0 B 1.0 to 8.0 Ni 65 to 90 Zr0.05 max.

Ca 0.20 max. C 0.70 max.

2. An impact and corrosion resistant article comprising a baseconsisting essentially of molybdenum, and animpact resistant outersurface coating bonded to said base, said coating consisting essentiallyof a matrix of a nickelchromium alloy containing dispersed particles ofa nickelchromium alloy having a higher melting point than said matrix,both said matrix and said particles having a composition within thefollowing range:

Mn 1.0 max. Si 0.5 to 5.0 Fe 1.0 to 5.5 Cr 8.0 to 20.0 B 1.0 to 8.0 Ni65 to 90 Zr 0.05 max. Ca 0.20 max. C 0.70 max.

said coating having a thickness in the range from about 0.002 inch toabout 0.010 inch.

3. The method of increasing the corrosion resistance and impactresistance of a molybdenum base which comprises applying over said basea mixture of corrosion resistant nickel-chromium alloys of differingmelting points, each of the alloys in said mixture having an analysis inthe following range:

-Mn 1.0 max.

Si 0.5 to 5.0 Fe 1.0 to 5.5 Cr 8.0 to 20.0 B 1.0 to 8.0 Ni 65 to 90 Zr0.05 max. Ca 0.20 max. C 0.70 max.

each of said alloys constituting substantial percentages of the totalmixture, heating the resulting composite article to a temperature andfor a time suflicient to render the lower melting alloy in said mixturemolten, said temperature being insufiiciently high to melt the highermelting alloy of said mixture, maintaining said lower melting alloymolten until said alloy reacts with the underlying surface and dissolvessome of said higher melting alloy, terminating said heating before allof said higher melting alloy is dissolved and cooling the compositearticle to provide a uniform, smooth coating over said molybdenum baseconsisting of a matrix of said lower melting alloy having dispersedtherein undissolved particles of said higher melting alloy.

4. The method of increasing the corrosion resistance and impactresistance of a molybdenum base which comprises applying over said basea mixture of corrosion resistant nickel-chromium alloys, each of thealloys in said mixture having an analysis in the following range:

Mn 1.0 max.

Si 0.5 to 5.0 Fe 1.0 to 5.5 Cr 8.0 to 20.0 B 1.0 to 8.0 Ni 65 to Zr 0.05max. Ca 0.20 max. C 0.70 max.

said mixture containing from 60 to 40% by weight of a higher meltingalloy and from 40 to 60% by weight of a lower melting alloy, heating theresulting composite article to a temperature and for a time sufficientto render the lower melting alloy in said mixture molten, saidtemperature being insufficiently high to melt the higher melting alloyof said mixture, maintaining said lower melting alloy molten until saidalloy reacts with the underlying surface and dissolves some of saidhigher melting alloy, terminating said heating before all of said highermelting alloy is dissolved and cooling the composite article to providea uniform, smooth coating over said molybdenum base consisting of amatrix of said lower melting alloy having dispersed therein undissolvedparticles of said higher melting alloy.

5. The method of increasing the corrosion resistance and impactresistance of a molybdenum base Which comprises chromizing saidmolybdenum base, applying over the resulting chromized surface a mixtureof corrosionresistant nickel-chromium alloys of differing meltingpoints, each of the alloys in said mixture having an analysis in thefollowing range:

Mn- 1.0 max. Si 0.5 to 5.0 Fe 1.0 to 5.5 Crn 8.0 to 20.0 B 1.0 to 8.0 Ni65 to 90 Zr 0.05 max. Ca 0.20 max. C 0.70 max.

each of said alloys constituting substantial percentages of the totalmixture, heating the resulting composite article to a temperature andfor a time sufficient to render the lower melting alloy in said mixturemolten, said temperature being insufiiciently high to melt the highermelting alloy of said mixture, maintaining said lower melting alloymolten until said alloy reacts with the underlying surface and dissolvessome of said higher melting V alloy, terminating said heating before allof said higher melting alloy is dissolved and cooling the compositearticle to provide a uniform, smooth coating over said molybdenum baseconsisting of a matrix of said lower melting alloy having dispersedtherein undissolved particles of said higher melting alloy.

References Cited in the file of this patent UNITED STATES PATENTS

1. AN IMPACT AND CORROSION RESISTANT ARTICLE COMPRISING A BASECONSISTING ESSENTIALLY OF MOLYBDENUM, AND AN IMPACT RESISTANT OUTERSURFACE COATING BONDED TO SAID BASE, SAID COATING CONSISTING ESSENTIALLYOF A MATRIX OF A NICKELCHROMIUM ALLOY CONTAINING DISPERSED PARTICLES OFA NICKELCHROMIUM ALLOY HAVING A HIGHER MELTING POINT THAN SAID MATRIX,BOTH SAID MATRIX AND SAID PARTICLES HAVING A COMPOSITION WITHIN THEFOLLOWING RANGE: